The present invention relates to a feedforward (FF) amplifier for compensating for distortion produced in an amplifier and, more particularly, to a feedforward amplifier for effectively providing phase control in a vector adjuster.
In a wireless communication system such as a mobile communication system, distortion produced when a signal to be sent is amplified by an amplifier in a base station unit or the like is compensated for by a feedforward type distortion compensator.
FIG. 4 shows an example of configuration of a fundamental circuit of a feedforward amplifier that compensates for distortion by a feedforward method. For convenience of illustration, configuration portions of FIG. 4 which are similar to their counterparts of FIG. 1 that will be referenced in an embodiment described later are indicated by the same reference numerals as in FIG. 1. It is to be understood, however, this does not restrict the scope of the present invention unnecessarily.
The feedforward amplifier shown in FIG. 4 has three directional couplers (combiner/splitter devices) HYB1, HYB2, and HYB3. Two routes are present between the directional couplers HYB1 and HYB2. One of the two routes has a variable attenuator AT1_1, a variable phase shifter PH1_1, and a main amplifier AMP1. The other route has a coaxial delay line D1. Similarly, two routes are present between the directional couplers HYB2 and HYB3. One of these two routes has a coaxial delay line D2. The other route has a variable attenuator AT2_1, a variable phase shifter PH2_1, and an auxiliary amplifier AMP2. The feedforward amplifier further includes a control portion 11 for controlling the two variable attenuators AT1_1 and AT2_1 and the two variable phase shifters PH1_1 and PH2_1.
The feedforward amplifier is composed of two loops, i.e., a distortion detection loop L1 and a distortion compensation loop L2. The detection loop L1 is made up of two directional couplers HYB1, HYB2 and intervening components, i.e., variable attenuator AT1_1, variable phase shifter PH1_1, main amplifier AMP1, and coaxial delay line D1. The compensation loop L2 is made up of two directional couplers HYB2, HYB3 and intervening components, i.e., coaxial delay line D2, variable attenuator AT2_1, variable phase shifter PH2_1, and auxiliary amplifier AMP2.
In each of the loops L1 and L2, the gain can be varied by the variable attenuator AT1_1 or AT2_1 such that the amplifier side route and the delay line route are identical in amount of delay and gain but are 180° out of phase with each other for a signal to be treated. The phase can be varied by the variable phase shifter PH1_1 or PH2_1. Such variations in gain and phase are controlled by the control portion 11. Generally, the control portion 11 monitors the output levels from the directional couplers HYB2 and HYB3 and controls the variable attenuators and variable phase shifters so as to maximize or minimize the output levels. This is known as adaptive control.
In each of the loops L1 and L2, the gain or phase is adjusted by the variable attenuator AT1_1 or AT2_1 or by the variable phase shifter PH1_1 or PH2_1. This is known as vector adjustment.